1. Technical Field
The invention relates to permeable base transistors.
2. Discussion of Related Art
Permeable base transistors (PBTs) offer advantages in speed and packing density over conventional field effect transistors (FETs) (Wernersson et al., Mat. Sci. and Eng. B 51:76-80 (1998); Nilsson et al., Solid State Elec. 42:297-305 (1998)). In typical PBT technologies, a metallic base layer is overlaid onto a single crystal semiconductor substrate (emitter/collector) to form a Schottky barrier (e.g., U.S. Pat. No. 4,378,629). A second epitaxial semiconductor layer is overgrown on the base layer (second collector/emitter). The base layer is patterned with openings so that current can flow from emitter to collector only when a voltage is applied to the base layer. A variety of metals, such as tungsten and metal silicides (e.g., WSi2, NiSi2 and CoSi2) have been used as materials for the base layer (von Kxc3xa4inel, Mat. Sci. Rep. 8:193-269 (1992); Zaring et al., Rep. Progress Phys. 56:1397-1467 (1993); Pisch et al., J. App. Phys. 80:2742-2748 (1996)). These PBTs were predicted to have high gains at very high frequencies (200 GHz), which were not achievable with conventional FET technologies. However, problems such as poisoning of PBT semiconductor structures by metal electromigration, insufficient heat dissipation, and complexity of epitaxial overgrowth to form embedded metal base layers have prevented mass fabrication of PBTs (Hsu et al., J. App. Phys. 69:4282-4285; Miyao et al., J. Cryst. Growth 111:957-960 (1991)).
Therefore, a need exists for new PBTs that provide the predicted improvements in speed, packing density, and high frequency performance over FETs, without suffering from the drawbacks associated with the metal base layers of traditional PBTs.
The invention provides a permeable base transistor (PBT) having a base layer that includes nanotubes. One aspect of the invention provides a permeable base transistor having a semiconductor emitter, a semiconductor collector, and a base in contact with the emitter and collector. The base includes metallic nanotubes. In at least some embodiments, the base includes an aggregate of carbon nanotube segments. Carbon nanotube segments contact other carbon nanotube segments to define a plurality of conductive pathways. In certain embodiments, the nanotubes include single-walled carbon nanotubes. In particular embodiments, the base includes a monolayer of nanotubes. In some embodiments, the base includes a patterned layer of nanotubes. In certain embodiments, the PBT also has an ohmic contact in communication with the emitter. In particular embodiments, the PBT also has an ohmic contact in communication with the collector.